The present disclosure relates to a lighting system in which an optical device employing an electrowetting phenomenon is embedded.
In recent years, development of optical devices employing an electrowetting phenomenon (electrocapillarity) has been advanced. The term “electrowetting phenomenon” refers to a phenomenon wherein when applying voltage between liquid having electroconductivity and an electrode, energy at the solid-liquid interface between the electrode surface and the liquid change, and the shape of the liquid surface changes.
FIGS. 68A and 68B illustrate principle diagrams for describing electrocapillarity. As schematically illustrated in FIG. 68A, for example, let us say that an insulating film 402 is formed on the surface of an electrode 401, and an electroconductive droplet 403 made up of an electrolytic solution is disposed on this insulating film 402. The surface of the insulating film 402 is subjected to water-repellent processing, and as shown in FIG. 68A, in a state in which voltage is not applied thereto, interaction energy between the surface of the insulating film 402 and droplet 403 is low, and a contact angle θ0 is great. Here, the contact angle θ0 is an angle which includes the surface of the insulating film 402 and the tangent line of the droplet 403, and depends on physical properties such as the surface tension of the droplet 403, the surface energy of the insulating film 402, and so forth.
On the other hand, as schematically illustrated in FIG. 68B, upon voltage being applied between the electrode 401 and droplet 403, electrolytic ions on the droplet side are concentrated on the surface of the insulating film 402, thereby causing change in electrification quantity of a charge double layer, and also inducing change in the surface tension of the droplet 403. This phenomenon is an electrowetting phenomenon, wherein the contact angle θV of the droplet 403 is changed due to the magnitude of applied voltage. That is to say, in FIG. 68B, the contact angle θV is represented with the following Expression (A), i.e., the Lippman-Young expression as the function of applied voltage V.cos(θV)=cos(θ0)+(½)(∈0·∈)/(γLG·t)V2  (A)
Here, the respective variables within the above-mentioned expression represent the following.
∈0: dielectric constant of vacuum
∈: specific inductive capacity of insulating film
γLG: surface intension of electrolytic solution
t: film thickness of insulating film
As described above, the surface shape (curvature) of the droplet 403 is changed depending on the magnitude of the voltage V to be applied between the electrode 401 and droplet 403. Accordingly, in the case of employing the droplet 403 as a lens element, an optical element of which the focal position (focal distance) can electrically be controlled can be realized.
Development of optical devices employing such an optical element has been advanced. For example, a lens array for strobe device has been proposed in Japanese Unexamined Patent Application Publication No. 2000-356708. With this lens array, insulating liquid droplets disposed on a water-repellent film of a substrate surface in an array manner and electroconductive liquid are encapsulated, thereby making up variable-focus lenses. Subsequently, each of the lenses is formed with an interface shape between insulating liquid and electroconductive liquid, the each lens shape is electrically controlled employing an electrowetting phenomenon, thereby changing focal distance. Also, a cylindrical lens made up of a liquid lens has been disclosed in Japanese Unexamined Patent Application Publication No. 2002-162507.
Note however, with the lens array disclosed in Japanese Unexamined Patent Application Publication No. 2000-356708, it is difficult to obtain great optical power. That is to say, even if the curvature of the lens is changed from a concave state to a flat state by turning on/off applied voltage, for example, the variable rate of the guide number of a strobe device is 1.48, and accordingly, a great variable rate is not readily obtained. Note that the details thereof will be described later.
To this end, it has been recognized that there is a demand to provide a lighting system in which an optical device, which is made up of a liquid lens employing an electrowetting phenomenon, having an arrangement and configuration whereby high optical power can be obtained.